The present invention relates to an optical information recording medium on/from which information is recorded/erased by using a change in optical constant upon a phase change induced by different thermal hystereses and, more particularly, to a phase change optical disk.
As an optical information recording medium on/from which information is recorded/erased by irradiating a laser beam, a magneto-optical disk, a phase change optical disk, and the like are known. For example, in the phase change optical disk, as shown in FIG. 4, a first dielectric layer 2, a recording layer 3, a second dielectric layer 4, and a reflecting layer 5 are sequentially formed on a substrate 1 to constitute a four-layer structure. Information is recorded and erased by using a phase change between the amorphous and crystalline states of the recording layer which is induced by different thermal hystereses of heating and cooling performed by irradiation of a laser beam. More specifically, the recording layer is fused and rapidly cooled to be brought to the amorphous state, thereby recording information. In contrast to this, the recording layer is kept at a crystallization temperature or more for a predetermined period of time to be brought to the crystalline state, thereby erasing information. A signal is reproduced by using the difference in reflectance between the amorphous state and the crystalline state. The thicknesses of the first dielectric layer 2, the recording layer 3, the second dielectric layer 4, and the reflecting layer 5 are optimized in terms of sensitivity, C/N ratio, erasability, rewritable repetition count, and the like. For the first and second dielectric layers 2 and 4, ZnS--SiO.sub.2, SiN, SiO.sub.2, or the like is used. For the recording layer 3, SbTe, GeSbTe, AgInSbTe, or the like is used. For the reflecting layer 5, Al, Au, or the like is used.
As a method of increasing a recording density, a mark edge recording method is effective. In this method, pieces of information are given to two ends of each recorded mark. In the phase change optical disk from which a signal is reproduced by using the difference in reflectance between the amorphous state and the crystalline state, the absorbance (Ac) in the crystalline state is often different from the absorbance (Aa) in the amorphous state. In general, Aa is higher than Ac. In such a case, the width or length of a mark to be formed is influenced by the state of the recording layer before an overwrite operation, i.e., the crystalline or amorphous state. That is, the jitter greatly increases after the overwrite operation. For this reason, in order to reduce the jitter and realize the mark edge recording method in the phase change optical disk, the sensitivity in the amorphous state must be set to be equal to that in the crystalline state. The disk is preferably designed such that Ac is higher than Aa, considering that the heat conductivity in the crystalline state is higher than that in the amorphous state, and the amount of latent heat upon fusion is large. As effective means for providing such a medium, the absorbance control method disclosed in Japanese Patent Laid-Open No. 1-149238 and the absorbance control method disclosed in Japanese Patent Laid-Open No. 4-102243 are known. The former method uses a transparent thin metal film as a reflecting film. The latter method uses a high-refractive-index material such as Si as a reflecting film. In these methods, it is important to greatly change the transmittance between the amorphous state and the crystalline state.
In addition, the method disclosed in Japanese Patent Laid-Open No. 6-4903 is known as an absorbance control method using a metal reflecting film.
As an optical head light source for a currently available optical disk, a laser diode with a wavelength of 780 to 830 nm is used. Attempts have been made to decrease the wavelength of light from a laser diode, and a high-power red laser diode with a wavelength of about 690 nm is being put into practice. A change in optical constant due to a phase change of a recording layer used for a phase change optical disk, i.e., the difference in optical constant between the amorphous state and the crystalline state, decreases with a decrease in wavelength. For this reason, in a medium having a conventional structure designed to perform absorbance control by using transmitted light, if the absorbance Ac is set to be higher than the absorbance Aa in the amorphous state, the difference in reflectance between the two states decreases, resulting in a decrease in C/N ratio. This problem becomes serious especially under the condition of a high linear velocity, in which Ac/Aa must be maximized. With the direct application of the above conventional techniques (Japanese Patent Laid-Open Nos. 1-149238, 4-102243, and 6-4903), it is difficult to obtain good overwrite characteristics. For example, Ge.sub.2 Sb.sub.2 Te.sub.5 exhibits the following optical constants (a refractive index n and an extinction coefficient k) at a wavelength of 830 nm: (n=4.6; k=1.06) in the amorphous state; and (n=5.89; k=3.47) in the crystalline state. At a wavelength of 690 nm, (n=4.36; k=1.72) in the amorphous state; and (n=4.46; k=4.0) in the crystalline state. That is, the change in the refractive index n is small. For this reason, as shown in FIG. 3, if the difference in reflectance between the crystalline and the amorphous state is set to be about 15% at a wavelength of 690 nm, Ac/Aa is about 1.1 at maximum. Therefore, under the condition of a high linear velocity, in which Ac/Aa needs to be large, an overwrite operation will increase the jitter. At a wavelength of 532 nm, which allows higher recording densities, although not practical at present, the changes in optical constants further decrease as follows: (n=3.53; k=1.98) in the amorphous state; and (n=2.39; k=3.86) in the crystalline state. Consequently, it is more difficult to perform absorbance control while ensuring a high C/N ratio.